In a UMTS (Universal Mobile Telecommunications System) network, the specifications of long-term evolution (LTE) have been drafted for the purpose of further increasing high-speed data rates, providing lower delay and so on (see non-patent literature 1). Also, successor systems of LTE (referred to as, for example, “LTE-advanced,” “FRA (Future Radio Access)” and so on (hereinafter referred to as “LTE-A”)) have been under study for the purpose of achieving further broadbandization and increased speed beyond LTE.
In LTE-A and later mobile communication systems, application of the MIMO transmission scheme (massive MIMO (Multi Input Multi Output)), which uses a large number of antenna elements in a high frequency band (hereinafter also referred to as “massive-MIMO”), is under study.
The massive-MIMO transmission scheme achieves improved data rates (spectral efficiency) by transmitting data using a large number (for example, 100 or more) of antenna elements. Since data is transmitted by using a large number of antenna elements, compared to when a small number of antenna elements are used, it becomes possible to improve the efficiency of transmission by virtue of multiplexing, and enable faster radio communication than heretofore. Also, reliable beamforming is made possible by combining a large number of antenna elements.
Here, “beamforming (BF)” refers to the technique of allowing transmitting/receiving beams to have directivity, and changing the shape of beams by controlling the amplitude and phase of each transmitting/received signal in a plurality of antenna elements. In this beamforming, generally, a larger number of antenna elements allow more reliable control. In other words, it is possible to control, in detail, the number of beams, the shape of each beam (the beam width in the horizontal plane, the beam width in the vertical plane and so on) and the direction and gain of beams, depending on the number of antenna elements. For example, high gain (power density) can be achieved by narrowing the width of beams (that is, by forming thin beams).